ROMA - It was a crude and savage killing, as five "clients" of an immigrant smuggler allegedly beat the Mexican youth to death in the brushy thickets outside of town.

His clients, authorities say, were members of the infamous Mara Salvatrucha gang, or MS-13, and were apparently angry at the 18-year-old smuggler for leaving behind a fellow gang member who had caused trouble.

The killing was in keeping with the violent reputation of the Central American street gang, whose members are known to be involved in various criminal enterprises.

''He was beaten to death with sticks and stones, and I'm not talking about little stones," said Marco Treviño, an assistant district attorney in Starr County. The prosecutor said two gang members are being held on murder charges related to the Aug. 10 attack and ''two or three" other gang members are being sought.

Across the Rio Grande Valley, local police are on the lookout for other MS-13 members, as Border Patrol agents report catching increasing numbers of gang members as they cross the river with other undocumented immigrants. Many are identified as MS-13 members because of large tattoos featuring gang initials in Gothic letters on their arms, hands, legs, chest and foreheads.

Federal agencies began a nationwide operation to round up MS-13 members and other gangs in March in response to a series of machete attacks in communities around Washington, D.C.

''Whenever we come in contact with illegals, we question them to see if they are gang members," said Sgt. Carlos Zamarron of the La Joya Police Department. He said the long-standing information about MS-13 members slipping across the Texas border with groups of undocumented workers has taken on new importance since the smuggler's death two weeks ago.

''We're very aware, and it's a big concern," Zamarron said.

Increased vigilance

In the two southernmost Border Patrol sectors on the Texas border — headquartered in McAllen and Laredo — agents have arrested 96 members of the MS-13 gang so far this fiscal year, which began Oct. 1, 2004, an agency spokesman said.

Although figures for the entire U.S.-Mexico border were not immediately available Thursday, most of the border arrests of MS-13 members have taken place in South Texas because of its proximity to Central America.

''They have a history of being very violent, and they are part of an organized criminal network heavily involved in human smuggling and drug smuggling," said Roy Cervantes, a spokesman for the Rio Grande Valley sector, where 48 gang members have been detained. ''They normally don't stay in the Valley; they use it as a corridor as they enter the United States and attempt to go to larger cities."

Border Patrol agents in the Rio Grande Valley sector arrested six gang members last year and caught only one in 2003, Cervantes said.

Cervantes said MS-13 members are traveling among large groups of immigrants from countries other than Mexico, which the agency refers to as ''OTMs." He stressed that gang members are placed in detention pending criminal proceedings or deportation.

''The Rio Grande Valley sector is an established corridor for smugglers that are moving OTMs into the interior, primarily because of geography, the (large) population and established smuggling networks on both sides of border," Cervantes said.

And in the sprawling Valley sector, which covers 316 miles of river from Brownsville to Roma, agents are now detaining more ''OTMs" than Mexicans, Cervantes said.

Motivated by money

So far this fiscal year, Border Patrol agents in the Valley sector have apprehended 119,470 immigrants, and 59 percent, or 70,305, were from countries other than Mexico, according to statistics provided by Cervantes.

In 2004, OTMs comprised 26,437 of the 96,641 detained, up from 14,919 OTMs among a total 81,185 immigrants detained in 2003, he said.

Gang members are telling Border Patrol agents they are leaving Central America because of hard economic times, the same motivation for most immigrants, Cervantes said.

''What we're seeing is the increase in MS-13 apprehensions is related to high numbers of OTMs in the sector," he said. ''The MS-13 members, in interviews with our agents, reveal they are departing from Honduras and Salvador ... for the same reasons others are leaving those areas — for economic reasons."

One high-profile arrest was made last February near Falfurrias by sheriff's deputies when they detained Lester Rivera Paz, 29, the reputed head of the MS-13 gang in Honduras.

Rivera fled the Central American country after being implicated in a Dec. 23 bus massacre in San Pedro Sula in which 28 people were shot to death.

The alleged gang leader was identified by the Border Patrol after his fingerprints were submitted to a government database, and has since been indicted on charges he entered the country illegally after four previous deportations. He remains in jail.

The roots of MS-13

The Mara Salvatrucha was formed in the early 1980s in Los Angeles by Salvadoran immigrants who had fled their homeland's brutal civil war.

Having their own gang allowed the Salvadorans — later joined by Hondurans, Guatemalans and other migrants — to protect themselves against the Mexican street gangs who dominated Los Angeles at the time.

Mara is Central American slang for street gang, the word deriving from a species of army ant that can devastate a forest.

The influx of Central American gang members worries border law officials, who are already combating crimes including kidnappings, drug trafficking and home invasions.

"This office is very concerned," said Treviño, the Starr County assistant prosecutor.